path: root/arch
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authorPeter Zijlstra <peterz@infradead.org>2020-10-27 19:33:30 +0100
committerThomas Gleixner <tglx@linutronix.de>2020-10-27 23:15:24 +0100
commitcb05143bdf428f280a5d519c82abf196d7871c11 (patch)
tree6ef08abfc2e82060af40c487089dc549597949dc /arch
parenta195f3d4528a2f88d6f986f6b1101775ad4891cf (diff)
x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
Commit d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6") changed the semantics of the variable from random collection of bits, to exactly only those bits that ptrace() needs. Unfortunately this lost DR_STEP for PTRACE_{BLOCK,SINGLE}STEP. Furthermore, it turns out that userspace expects DR_STEP to be unconditionally available, even for manual TF usage outside of PTRACE_{BLOCK,SINGLE}_STEP. Fixes: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6") Reported-by: Kyle Huey <me@kylehuey.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Kyle Huey <me@kylehuey.com> Link: https://lore.kernel.org/r/20201027183330.GM2628@hirez.programming.kicks-ass.net
Diffstat (limited to 'arch')
1 files changed, 6 insertions, 3 deletions
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 32b2d39272e0..e19df6cde35d 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -937,10 +937,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
- * Clear the virtual DR6 value, ptrace() routines will set bits here
- * for things it wants signals for.
+ * Start the virtual/ptrace DR6 value with just the DR_STEP mask
+ * of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
+ *
+ * Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
+ * even if it is not the result of PTRACE_SINGLESTEP.
- current->thread.virtual_dr6 = 0;
+ current->thread.virtual_dr6 = (dr6 & DR_STEP);
* The SDM says "The processor clears the BTF flag when it

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